FIG. 1 shows an example of a known gas turbine engine 10 having a compressor section 12, a combustor section 14 and a turbine section 16. In the turbine section 16, there are alternating rows of stationary airfoils 18 (commonly referred to as vanes) and rotating airfoils 20 (commonly referred to as blades). Each row of blades 20 is formed by a circular array of airfoils 20 attached to a disc 22 provided on a rotor 24 with an axis 59. The blades 20 extend radially outward from the discs 22 and terminate in blade tips 26. Herein, the terms “axial,” “radial” and “circumferential” and variations thereof are intended to mean relative to the turbine axis 59. Each row of vanes 18 is formed by attaching a circular array of vanes 18 to a stationary vane carrier 28. The vanes 18 extend radially inward from the inner surface 30 of the vane carrier 28. The vane carrier 28 is attached to an outer casing 32, which encloses the turbine section 16 of the engine 10.
Between the rows of vanes 18, a ring seal 34 is attached to the inner surface 30 of the vane carrier 28. The ring seal 34 is a stationary component that acts as a hot gas path guide between the rows of vanes 18 at the locations of the rotating blades 20. The ring seal 34 is commonly formed by a plurality of ring segments. The ring segments are attached either directly to the vane carrier 28 or indirectly such as by attachment to metal isolation rings that attach to the vane carrier 28. Each ring seal 34 surrounds an array of blades 20 such that the tips 26 of the rotating blades 20 are in close proximity to the ring seal 34.
During engine operation, high temperature, high velocity gases 71 flow generally axially with respect to the turbine axis 59 through the rows of vanes 18 and blades 20 in the turbine section 16. The ring seals 34 are exposed to these gases. In order to withstand the high temperature, ring seals 34 may be cooled by a diverted portion of compressed intake air from the compressor 12. Demands to improve engine performance have been met in part by increasing engine firing temperatures. For this reason the ring seals 34 have been made of ceramic matrix composites (CMC), which have higher temperature capabilities than metal alloys. By utilizing such materials, cooling air can be reduced, improving engine performance, emission control and operating economics. Laminated CMC materials, both oxide and non-oxide based, have anisotropic strength properties. The interlaminar tensile and shear strengths (the through-thickness strengths) of CMC are substantially less than its in-plane strength.
CMC ring segments are typically attached to metal support structures outside the gas path. As a result, some of the CMC features are situated out-of-plane of the base plate; that is, some fibers of the CMC material are not parallel to the wall surface exposed to the hot gas path. Flanges or walls extend normally outward from the base plate to provide both rigidity and mounting points. During engine operation, differential pressure loads and other mechanical loads are reacted by these out-of-plane features.
Ring seal segments may be cooled by supplying a pressurized coolant such as air to the backside or “cold” side of the ring seal segment, which is its radially outer side. This coolant is supplied at a greater pressure than the hot gases 71 flowing through the turbine section in order to prevent the hot gas from leaking outward between the segments or into the cooled cavity. As a result, ring seal segments are subjected to pressure loading that is transmitted to the attachment points through the out-of-plane CMC features. This load passes through the intersections or transition regions between the out-of-plane features and the hot gas sealing wall—generally at 90° fiber corners and fillets where the CMC may be weakest. Such areas tend to be design-limiting features of these components.
A ring segment may be formed with a base plate and a frame of walls extending outwardly from the periphery of the base plate. However, such simple box-constructed ring segments are limited by interlaminar shear around the perimeter of the base plate. This shear is due to the pressure-induced normal force on the base plate from the cooling gas. A current approach is to simply thicken the walls and base plate until shear criteria are met. This is sufficient for some land-based power generation applications. However it is less practical as the pressure differential increases for efficiency in advanced engine designs, and it is undesirable in aero engines due to weight.
Isogrid structures are used in the aerospace industry for stiffening aircraft and spacecraft skins, engine casings, etc. These isogrids are optimized to minimize weight while maximizing bending stiffness to avoid buckling. An isogrid may be defined as a lattice of intersecting ribs forming an array of triangles, especially equilateral triangles. An orthogrid is a lattice of intersecting ribs forming an array of rectangles.